The present invention relates to the field of organic surface coatings, the said coatings being in the form of organic films. It relates more particularly to the use of a family of molecules suitably selected in order to make possible the simple and reproducible formation of ultrathin organic films (that is to say, the thickness of which is generally less than about ten nanometers or composed only of a few monomeric layers) by electrochemical grafting to electrically conducting or semiconducting surfaces.
There currently exists several techniques which make it possible to prepare thin organic films on substrates, each being based on an appropriate family or class of molecules.
The processes for forming a coating by centrifuging, known under the term of “spin coating” (or the related techniques for forming coatings by immersion (dip coating) or by deposition by spraying (spray coating)) do not require a particular affinity between the molecules deposited and the substrate of interest. This is because the cohesion of the deposited film is based essentially on the interactions between the constituents of the film, which can, for example, be crosslinked after deposition in order to improve the stability thereof. These techniques are highly versatile, can be applied to all types of surfaces to be covered and are highly reproducible. However, they do not make possible any effective grafting between the film and the substrate (simple physisorption is involved) and the thicknesses produced are always greater than about ten nanometers. Moreover, the spin coating techniques only make possible uniform deposited layers when the surface to be covered is essentially flat (French Patent Application FR-A-2 843 757). The minimum thicknesses accessible to the spray coating techniques are related to the wetting of the surfaces by the sprayed liquid, since the deposited layer only becomes essentially film-forming when the drops coalesce. Finally, the thickness of the deposited layers obtained by dip coating depends in a rather complex fashion on a certain number of parameters, such as the viscosity of the dipping liquid, and on the process (withdrawal rate).
Other techniques for forming an organic coating at the surface of a support, such as plasma deposition, for example described in the papers by Konuma M., “Film deposition by plasma techniques”, (1992) Springer Verlag, Berlin, and by Biederman H. and Osada Y., “Plasma polymerization processes”, 1992, Elsevier, Amsterdam, or else photochemical activation, are based on the same principle: generating unstable forms of a precursor close to the surface to be covered, which unstable forms change with the formation of a film on the substrate. While plasma deposition does not require any specific property of its precursors, photoactivation for its part requires the use of photosensitive precursors, the structure of which changes under irradiation with light. These techniques generally give rise to the formation of adherent films, although it is generally impossible to discern whether this adhesion is due to crosslinking of a film topologically closed around the object or to true formation of bonds at the interface.
The self-assembling of monolayers is a technique which is very simple to employ (Ulman A., “An introduction to ultrathin organic films from Langmuir-Blodgett films to self-assembly”, 1991, Boston, Academic Press). However, this technique requires the use of generally molecular precursors having a sufficient affinity for the surface of interest to be coated. The term used will then be “precursor-surface pair”, such as sulphur compounds having affinity for gold or silver, trihalosilanes for oxides, such as silica or alumina, or polyaromatics for graphite or carbon nanotubes. In all cases, the formation of the film is based on a specific chemical reaction between a part of the precursor molecule (the sulphur atom in the case of the thiols, for example) and certain “receptor” sites of the surface. A chemisorption reaction provides the attachment. Thus, at ambient temperature and in solutions, films with the thickness of a molecule (less than 10 nm) are obtained. However, while the pairs involving oxide surfaces give rise to the formation of very firmly grafted films (the Si—O bond involved in the chemisorption of trihalo-silanes on silica is among the most stable in chemistry), this is not at all the case when oxide-free metals or semiconductors are involved. In these cases, the interfacial bonding between the conducting surface and the monomolecular film is weak. Thus, the self-assembled monolayers of thiols on gold desorb as soon as they are heated above 60° C. or in the presence of a good solvent at ambient temperature or as soon as they are brought into contact with an oxidizing or reducing liquid medium. Similarly, Si—O—Si bonds are weakened immediately they are in an aqueous medium, indeed even a humid medium, in particular under the effect of heat.
The electrografting of polymers is a technique based on the initiation and then the polymerization, by chain propagation, which is electroinduced of electroactive monomers on the surface of interest, which acts both as electrode and as polymerization initiator (S. Palacin et al., “Molecule-to-metal bonds: electrografting polymers on conducting surfaces”, ChemPhysChem, 2004, 10, 1468). Electrografting requires the use of precursors suited to its mechanism of initiation by reduction and of propagation, generally anionic, as preference is often given to cathodically initiated electrografting, which can be applied to noble and non-noble metals (in contrast to electrografting by anodic polarization, which can be applied only to noble substrates). “Depleted vinyl” molecules, that is to say vinyl molecules carrying electron-withdrawing functional groups, such as acrylonitriles, acrylates, vinylpyridines, and the like, are particularly suitable for this process, which gives rise to numerous applications in the field of microelectronics or the biomedical field. The adhesion of the electrografted films is provided by a carbon-metal covalent bond. However, the polymerizable nature of the precursor results in relatively thick electrografted films, that is to say having a thickness rarely of less than 50 nm.
According to this electrografting technique, the polymerization is essential for the formation of the carbon/metal interfacial bond: this is because it has been shown (G. Deniau et al., “Coupled chemistry revisited in the tentative cathodic electro-polymerization of 2-butenenitrile”, Journal of Electro-analytical Chemistry, 1998, 451, 145-161) that the mechanism of electrografting proceeds by an electro-reduction of the monomer on the surface to give an unstable radical anion which, if it were not in the middle of polymerizable molecules, would desorb to return into solution (op.cit.). Aside from this desorption reaction, the addition reaction (of Michael addition type) of the charge of the first chemisorbed radical anion to a free monomer offers a second means of stabilizing the reaction intermediate: the product of this addition again gives a radical anion, where the charge has, however, “moved away” from the surface, which contributes to stabilizing the adsorbed structure. This dimeric radical anion can itself again add to a free monomer, and so on: each new addition is an additional stability by relaxation of the charge/polarized surface repulsion, which amounts to saying that the interfacial bond of the first radical anion, which is temporary, becomes stable as the polymerization takes place. In other words, it has been put forward that a vinyl monomer which cannot polymerize cannot be electrografted.
Among the various techniques recalled above, electrografting is the only technique which makes it possible to produce grafted films with specific control of the interfacial bonding. Moreover, contrary to the plasma or photoinduced techniques, electrografting only generates its reactive entities in the immediate vicinity of the surface of interest (in the electrochemical double layer, the thickness of which is a few nanometers in the majority of cases).
It appears to be accepted today that the production of grafted polymer films by electrografting activated vinyl monomers to conducting surfaces proceeds by virtue of electroinitiation of the polymerization reaction starting from the surface, followed by growth of the chains monomer by monomer. The reaction mechanism for electrografting has in particular been described in the papers by C. Bureau et al., Macro-molecules, 1997, 30, 333; C. Bureau and J. Delhalle, Journal of Surface Analysis, 1999, 6(2), 159 and C. Bureau et al., Journal of Adhesion, 1996, 58, 101.
By way of example, the reaction mechanism for the electrografting of acrylonitrile by cathodic polarization can be represented by the following Scheme A:

In this scheme, the grafting reaction corresponds to stage 1, where growth takes place starting from the surface. Stage 2 is the main side reaction, which results in the production of an ungrafted polymer; this reaction is limited by the use of high concentrations of monomer.
The grafted chains thus grow by purely chemical polymerization, that is to say independently of the polarization of the conducting surface which gave rise to the grafting. This stage is thus sensitive to (and is in particular interrupted by) the presence of chemical inhibitors of this growth, in particular by protons.
In Scheme A above, where the electrografting of acrylonitrile under cathodic polarization was considered, the growth of the grafted chains takes place by anionic polymerization. This growth is interrupted in particular by protons and it has even been shown that the content of protons constitutes the major parameter which controls the formation of polymer in solution; the information obtained during synthesis, in particular the appearance of the voltamogrammes which accompany the synthesis, show it (see in particular the paper by C. Bureau, Journal of Electro-analytical Chemistry, 1999, 479, 43). Traces of water and more generally the labile protons from protic solvents constitute sources of protons which are harmful to the growth of the grafted chains.
The majority of vinyl compounds can be successfully grafted to conducting surfaces. However, it has been shown that it is impossible to form films starting from some vinyl compounds; more particularly, it has proved impossible, to date, to produce grafted polymers starting from crotononitrile.
One explanation for this reaction being impossible was put forward in the abovementioned paper by G. Deniau et al., J. Electroanal. Chem., 1998. This is because it has been shown that the —CH3 group of 2-butenenitrile (or crotononitrile), positioned in the cis or trans position with respect to the driving electron-withdrawing nitrile group, experiences a strong reduction in its pKa in comparison with that of the protons of the —CH3 group of methylpropenenitrile. In the case of methacrylonitrile, where the electron-withdrawing —CH3 and —CN radicals are carried by the same carbon atom, a conventional pKa of the order of 30 is observed in acetonitrile, indicating protons of very low acidity.
Thus, the pKa values of the —CH3 radicals for crotononitrile and methacrylonitrile were respectively calculated, in acetonitrile, according to the following reactions (1) and (2):

The results are given in Table I below:
TABLE IReaction No.ΔG/kJ · mol−1Calculated pKa1+11019.12+18432.1
These results unambiguously show that the protons of the —CH3 radical of crotononitrile make this molecule a weak acid, within the Brönsted sense, in acetonitrile and give an explanation for the nonpolymerization observed experimentally for this molecule.
Thus, in the case illustrated in the following reactions (3) and (4), it is observed, after the electrochemical reduction (reaction 3), that the radical anion formed reacts in solution with the acidic proton of a neutral molecule (reaction 4):

The overall reactivity of crotononitrile and of its radical anion formed after reduction is described in the paper by Deniau G. et al., 1998 (abovementioned), where it appears the crotononitrile does not make it possible to produce electrografted organic films.
Likewise, at the surface, the mechanism represented in the following Scheme B has been proposed, by analogy with what has been established for methacrylonitrile:

In this scheme, after the reaction of grafting to the surface, the anion heals not by a reaction of Michael type, as for methacrylonitrile (anionic propagation of the polymerization, as appears in Scheme A above), but by capturing an acidic proton from a neutral molecule.
In this Scheme B, reaction (1) reflects the possible tautomerism of the —CH3 radical of the grafted radical anion which can result in the following anion:Surface-CH(—CH2−)—CH2CN
It is only in water that the mobility of the protons is very substantially different from that of the other cations as the proton “does not move”. In organic solvents, it is a nucleus like others which has to actually move, and it makes sense to distinguish between an intra- and intermolecular proton transfer. This mechanism is therefore only possible in the case of molecules having an intramolecular proton.
Reaction (2) of Scheme B results in the same product but directly via an intermolecular route due to the relative acidity of the protons of the crotononitrile.
Overall, if it is therefore known how to produce chemical bonds on conducting or semiconducting substrates by electrografting various precursors, it remains difficult to obtain, by virtue of these reactions, ultrathin films approaching the monolayer (thickness<10 nm) as the underlying reaction mechanisms do not make it possible to satisfactorily control at scales of a nanometer or less. To date, only aryldiazonium salts have made possible a solution approach to this problem.
Thus, as disclosed, for example, in French Patent Application FR-A-2 804 973, the electrografting of precursors, such as aryldiazonium salts, which carry a positive charge takes place by virtue of a reaction for cleaving the salts in their reduced form to give a radical which is chemisorbed on the surface. Just as for the electrografting of the polymers, the reaction for the electrografting of aryldiazonium salts is electroinitiated and results in the formation of interfacial chemical bonds. Unlike the electrografting reactions of polymers, the electrografting of aryldiazonium salts does not “need” a coupled chemical reaction to stabilize the chemisorbed entity formed subsequent to the charge transfer as this entity is electrically neutral and not negatively charged, as in the case of a vinyl monomer. It thus results a priori in a stable surface/aryl group adduct.
However, it has been demonstrated, in particular in French Patent Application FR-A-2 829 046, that aryl-diazonium salts result in ultrathin organic films which are electrically conducting and which can thus grow on themselves: once the grafting to the initial surface has been carried out by an electrocleavage reaction+chemisorption, the film grows by an electro-controlled reaction in the manner of a conducting polymer film but at the cathode. This makes it difficult to control the thicknesses of the organic films resulting from the electrografting of aryldiazonium salts, in particular in the very low ranges of thickness, that is to say below 100 nm and in particular below 20 nm.
For the purpose of aiming at monolayers of aryl groups starting from aryldiazonium salts, the proposal has been made to limit the growth of the chains by minimizing the formation of radicals in the vicinity of the surface of the electrode by adding radical scavengers (James Tour et al., J. Am. Chem. Soc., 2004, 126, 370-378). As the growth of the film on itself is intrinsically a reaction with second order kinetics with respect to the concentration of aryl radicals, whereas the chemisorption and grafting reaction is first order with respect to the same concentration, it is then understood that the decrease in this concentration is in favour of better selectivity for the grafting at the expense of the growth.
However, it is generally observed that it is the combined kinetics which are first slowed down and advantageous degrees of coverage are then only obtained for very long times compatible with difficulty with a large number of industrial applications.
By extension, the addition of scavengers of growing ends for other monomeric substrates might be envisaged as a means of very rapidly terminating the growth in order to produce very short chains and thus to control at low thicknesses. Nevertheless, the concentration ratio of the “propagators” (monomers) to the growth “inhibitors” generally results only in random controlling of the lengths of growing chains, worthwhile with large numbers (i.e., high degrees of polymerization) but not in a position to provide a sufficiently precise parameter for control of the lengths of chains resulting in ultrathin layers (less than or equal to 10 nm).
Furthermore, the one-to-one relationship between thickness and length of chains is improper: the grafting density of the chains, which gives the number of bases of chains per unit of surface area, also has to be known in order for the relationship to be effectively one-to-one. This is because it is possible to produce a given thickness either with very dense chains (close packing), by controlling solely the length of the chains as a brush, but also at a given (possibly high) chain length by controlling the grafting density (at low density, the chains are “flattened” on the surface and give an apparent thickness which is lower than the chain length). Controlling the length of growing chains, without further reflections, is thus generally insufficient to control the thickness of the film obtained.